A city on the Moon?

Lunar transport vehicle, film 2001

Building a lunar colony is a favorite theme in science fiction. I would cite one of my favorite stories featuring such a colony: The Menace from Earth by Robert A. Heinlein (1957). A lunar colony also appears in the novel and film 2001: A Space Odyssey by Arthur C. Clarke.

Recently, with the return of humans to the Moon (still in its early stages, as astronauts have only orbited the Moon), there has been renewed discussion about the possibility of building a permanent lunar facility. There is even talk of establishing a city, with a population of over 100,000.

A piece in Physics World—referencing another article published in Frontiers in Space Technologies—addresses this issue by examining the needs of a lunar colony and the potential cost of maintaining it.

The most critical needs for a lunar colony would be as follows:

1.                  Materials: Many could be extracted from the lunar subsoil. The rest would have to be sent from Earth at a much higher cost.

2.                  Energy: There would be two options for generating power on the Moon itself: building a nuclear power plant or a solar power plant.

3.            Water: Since the lunar subsoil appears to be dry—as if baked during the satellite's history—the only available water source would be the deposits that seem to have accumulated in the Moon's permanently shadowed polar regions, where water can remain in a solid state (ice).

The article under discussion estimates that building a nuclear power plant on the Moon would not be necessary—even to supply energy to a city of one million inhabitants. Instead, a solar power plant consisting of kilometer-high towers covered in photovoltaic panels would be enough. This setup could generate approximately 3 GW of power, which should be adequate, at least according to current estimates of the energy requirements for such a lunar city. 

The water problem, however, is more difficult to solve. The most optimistic estimates suggest that the total amount of water at the lunar poles could approach one billion tons. How long would this amount of water last if used in a lunar city? It would depend on two factors:

a)                  The population. The more people living in the city, the higher the water consumption.

b)                  Water recycling for reuse. The problem here is that achieving a 100% recycling rate is impossible; some loss always occurs, however slight.

As an example of the latter, water recycling in the International Space Station is extraordinarily high, reaching 98%. Although the station has a small volume—making it easier to prevent losses—let us assume it is possible to achieve a similar recycling rate on the Moon.

One billion tons of water (the amount believed to exist on the Moon) seems like a vast quantity. Yet, without recycling, that supply would last only a couple of years in a city of one million inhabitants. If the lunar colony housed 100,000 people, it would last about 20 years.

The article under discussion calculates that, with a 98% recycling rate, all the water on the Moon would be depleted in a century, if it must supply a city of one million inhabitants. If recycling methods were improved beyond the level achieved on the space station, an efficiency of 99.75% would be required to sustain a lunar colony of that size for a much longer period.

With the 98% rate we are currently capable of achieving, the size of the lunar colony would need to be reduced to about 10,000 inhabitants.

The same post in Spanish

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Manuel Alfonseca